Following the synthesis of precursor transfer RNA from the DNA template several modification reactions occur to alter the size and base composition of the tRNA species. The objectives of my research program are to 1. Isolate mutants of Salmonella typhimurium lacking these modification functions. 2. Characterize these mutants to determine the physiological consequences of the alterations. 3. Define the biochemical nature of the lesion (e.g. examine the modified bases to determine if one is absent or produced in reduced quantities). 4. Analyze the biochemical properties of the altered tRNA species to determine if the alteration effects the amino-acylation reaction and/or ribosome binding. The initial stages of this program have been concerned with a detailed characterization of a cold-sensitive mutant which appears to have an altered tRNA maturation function. This mutant ceases growth rapidly upon shift to a low temperature and in addition has reduced levels of certain charged tRNA species even at high (permissive) temperature. Biochemical investigations are in progress to identify primary cause of these effects. Other mutants with phenotypes suggestive of tRNA maturation alterations are being characterized in conjunction with the work on the cold-sensitive mutant.